Hearing impairment is the most common human sensory disorder, involving as much as 50% of the population by age 80. Most cases of profound hearing impairment in children involve pathology of the inner ear, which contains the organs of auditory sensation. The normal development and function of the inner ear depends upon complex interactions between an extraordinarily diverse group of cell types, and thus is expected to involve many genes. The broad aim of this proposal is to identify and mutate genes involved in the development and/or function of the mouse inner ear in order to contribute to the molecular and genetic description of how these complex processes normally occur. In addition, we hope to generate mouse models of human inner ear dysfunction that will be helpful in understanding and potentially treating the underlying pathology. An induction gene trap screen designed to simultaneously mark and mutate endogenous genes in mouse embryonic stem cells by insertion of a lacZ reporter gene has provided an effective means of identifying genes expressed in the mouse inner ear. Mouse stains have been developed from these gene traps cell lines, and in one case, the lacZ insertion gave rise to dominant unilateral hearing loss. In t his appliation, it is proposed to refine and expand the in vitro portion of the gene trap screen by altering the induction conditions. The cell lines that express the reporter gene in vitro will be used to create chimeric embroyos which will be examined for reporter gene activity. Gene trap cell lines selected on the basis of reporter gene expression in the developing inner ear, molecular identify of the trapped gene, and map position of the trapped gene, will be used to generate mouse starins. Expression of the trapped genes will be followed throughout normal inner ear development by staining gene trap heterozygotes for beta-gal activity. Carriers of the gene trap insertions will be intercrossed and the offsrping assessed for changes in inner ear funtion and structure attributable to the vector insertion. Finally, to test the underlying hypothesis that the trapped genes are regulated in mice by the signals used to identify them in cultured cells, expression of the gene trap insertions will be examined in embryos in which the relevant signaling pathway has been perturbed by genetic or pharmacologic means.